An organic thin-film transistor may be formed by using a printing method when a material soluble in a solvent is used for a semiconductor material, an insulating material, and an electrode material, and for example, a large area organic thin-film transistor may be formed on a film substrate having flexibility inexpensively. Also, a thin film or circuit may be formed readily by using a printing method, a spin-coat method, a dipping method, or the like.
As one parameter for indicating a characteristic of such an organic thin-film transistor, there may be provided an on-off ratio of electric current (Ion/Ioff). In an organic thin-film transistor, an electric current Ids flowing between source and drain electrodes in a saturation region is represented by the formula:Ids=μCinW(VG−VTH)2/2L, wherein μ is a field-effect mobility, Cin is a capacitance per unit area of a gate insulating film, W is a channel width, L is a channel length, VG is a gate voltage, and VTH is a threshold voltage. Additionally, Cin is represented by the formula:Cin=∈∈0/d, wherein ∈ is a relative dielectric constant of the gate insulating film, ∈0 is the dielectric constant of vacuum, and d is a thickness of the gate insulating film.
Therefore, it is found that it is effective to increase the field-effect mobility, decrease the channel length, or increase the channel width, etc., in order to increase an on-state current. Herein, the field-effect mobility principally depends on material characteristics. Meanwhile, it has been known that the distance between source and drain electrodes is decreased in order to decrease the channel length.
The channel length is usually 10 μm or less because the field-effect mobility of an organic semiconductor material is smaller than that of a silicon semiconductor. As one method for decreasing the distance between source and drain electrodes accurately, there is provided photolithography used for forming a Si-based thin film transistor.
Photolithography is usually composed of processes of:
1. applying a photoresist layer on a substrate having a thin-film layer (resist application);
2. removing a solvent by means of heating (pre-bake);
3. conducting irradiation with ultraviolet light through a hard mask that has been patterned by using laser or electron beams in accordance with pattern data (light exposure);
4. removing the resist at a light-exposed portion with an alkaline solution (development);
5. curing the resist at a light-unexposed portion (patterned portion) by means of heating (post-bake);
6. conducting dipping in an etching liquid or exposure to an etching gas so as to remove the thin-film layer at a resist-free portion (etching); and
7. removing the resist by means of an alkaline solution or an oxygen radical (resist release). After each thin-film layer is thus formed, a thin-film transistor is formed by repeating the processes 1-7 but its expensive equipment and long-time process may cause its cost increase.
Therefore, an electrode pattern has been formed by using a printing method such as an ink-jet method or the like in order to reduce the production costs. As the ink-jet method is used, it may be possible to make an electrode pattern directly, and accordingly, the efficiency of material use may be increased, whereby it may be possible to realize simplification and cost reduction of a production process. In the ink-jet method, however, it may be difficult to reduce the degree of ejection, and it may be difficult to form a fine pattern, as a landing precision depending on a mechanical error or the like is taken into consideration.
Herein, Japanese Patent Application Publication No. 2006-060113 discloses a laminated structure including a wettability changing layer containing a material whose critical surface tension is changed by irradiation with ultraviolet rays, an electrically conductive layer, and a semiconductor layer. Then, the wettability changing layer has at least two portions with different critical surface tensions such as a higher surface energy portion with a greater critical surface tension and a lower surface energy portion with a smaller critical surface tension. Also, the electrically conductive layer is formed on the higher surface energy portion of the wettability changing layer by providing a liquid containing an electrically conductive material using an ink jet method while the semiconductor layer is provided, at least, so as to contact the lower surface energy portion of the wettability changing layer. Furthermore, the wettability changing layer contains a polymer material containing a methylene group and having a hydrophobic group at a side chain thereof. Thereby, it may be possible to readily form a fine patter.
Meanwhile, Japanese Patent Application No. 2006-134959 discloses a method for manufacturing an organic transistor composed of a substrate, a gate electrode, a two- or more-layer-laminated gate insulating layer, source and drain electrodes, and an organic semiconductor layer. Then, a channel is formed by applying mask light exposure with ultraviolet rays (UV light) in a wavelength range of 200 nm or greater and 300 nm or less on the two- or more-layer-laminated gate insulating layer, ejecting an electrode material for making the source and drain electrodes onto a mask-light-exposed portion by means of an ink jet method, and dividing the electrode material into a mask-light-exposed part and a light-unexposed part due to the difference between their surface free energies.
However, there may be a problem that the tact time is so long that simplification or cost reduction of a production process is hardly attained, because the irradiance of the ultraviolet rays is large for sufficiently changing the critical surface tension or surface free energy.